Many products require an optical system to spread light over a large area and control the direction of the light as it exits the system. Recent improvements in the performance of LEDs, coupled with a concurrent reduction in the cost of production, have made LEDs a more viable option for many applications. However, many applications; such as LCD backlights, signs with backlights, overhead lighting, and automotive lighting; require the concentrated light that is generated by an LED to be spread over a large area, while still controlling the direction of the light. These applications require an improved optic system to provide the desired light control.
Displays based on LCD technology have been evolving for decades. Numerous patent references based on improvements to the basic technology are now available. However, current art displays still have several shortcomings. The chief shortcoming of current art devices is excessive energy consumption. A 65″ diagonal HDTV LCD TV typically draws around a half of a kilowatt. This is a result of the poor efficiency of the technology.
One way to improve the efficiency of LCD displays is to direct as much as possible of the available light from the light source toward the area most easily seen by the viewer. With a hand held display device, where power consumption is clearly an important consideration, a narrowly angled light directed towards the viewer is desired.
In a standing application, such as a TV, it is desirable to have the highest intensity segment of the light projected in a direction normal to the surface of the display. It is also important to provide a significant amount of light to the left and right of normal. This is required for viewers that are not in the optimal (normal to the screen) viewing position. It is also desirable in these applications to reduce the amount of light that is projected above and below the angel normal to the screen. If the light that is typically directed in the off normal directions is re-directed to the preferred angles, the intensity of the light transmitted in the preferred directions would be greater.
Three groups of prior art references have addressed the control of light to LCD type displays. Among these, prism type “brightness enhancing films” (BEFs), comprise the most common class. One example of a BEF device is U.S. Pat. No. 5,467,208, “Liquid Crystal Display” by Shozo Kokawa, et al., issued Nov. 14, 1995. This reference discusses the prior art of prism type films and discloses improvements to the art. One drawback to prism films is that they have only limited control of the angles of the light output. Changes to the prism features result in only slight variations in the light output. The prism films are also limited to a two dimensional structure. If an application requires control of the light in three dimensions, at least two BEFs must be deployed.
A second class of prior art is exemplified by U.S. Pat. No. 6,421,103, “Liquid Crystal Display Apparatus . . . ” by Akira Yamaguchi, issued Jul. 16, 2002. The Yamaguchi reference discloses another device to control light as it enters an LCD panel. The patent discloses light sources, a substrate (not used as a light guide), apertures, and reflective regions on the substrate. The light is either reflected by the reflective surface or passes through the apertures. The light that passes through the apertures is captured by a lens used to control the direction of the light. Yamaguchi teaches restriction of the angle of the output light to concentrate more light directly at the viewer of an LCD type display. The Yamaguchi device provides much greater control of the output light than can be had with a BEF device. But a drawback to the Yamaguchi device is that it is extremely inefficient. Light must reflect off of the reflective surface many times before it exits the aperture. Even when the reflective surface is made with a high reflectance material, the losses in intensity are substantial. Therefore while the control of light with this invention is superior to that of BEF devices, the efficiency is much poorer.
U.S. Pat. No. 5,396,350, “Backlighting Apparatus . . . ” by Karl Beeson, issued Mar. 7, 1995; and U.S. Pat. No. 7,345,824, “Light Collimating Device” by Neil Lubart, issued Mar. 18, 2008; disclose devices in the third class of light control optics for LED light source devices. The Beeson and Lubart references disclose a reflective structure on the side of the light guide. The range of control of these reflective structures is limited, and is not equivalent to the control afforded by devices such as Yamaguchi. Further, the reflective structures are positioned very close to the LCD panel, which allows defects in their output to be easily seen by the viewer of the display.